1. Field of the Invention
The present invention relates to the field of photobioreactors, i.e. apparatuses permitting the controlled growth of photosynthetic microorganisms (microalgae, etc.).
2. Discussion of the Related Art
Conventionally, photobioreactors have positioned in series in a pipe closed on itself and traversed by a nutrient medium solution, a carbonator and a solar receptor. The carbonator is the point in the circuit where the solution is enriched with carbon dioxide gas and the solar receptor the point in the circuit where the nutrient solution is exposed to solar radiation in order to obtain the transformation of the carbon dioxide gas into biomass. This biochemical reaction corresponds to a photopolymerization of the carbon dioxide gas, which can be represented by the Myers equation: EQU 6.14CO.sub.2 +3.65H.sub.2 O+NH.sub.3 .fwdarw.C.sub.6.14 H.sub.10.3 O.sub.2.24 N+6.85O.sub.2
It is also known that such apparatuses roughly become dirty or contaminated unless special precautions are taken. Thus, adhesions of microalgae occur in a natural manner, particularly on the inner walls of the solar receptor pipes and the extent of this phenamenon is a function of the cultured alga species, as well as the constituent material of the tubes forming the solar receptor and the culturing conditions. In addition, the photosynthesis activity of the algal cells can lead to the appearance of gas pockets within the solar receptor pipes. Such an accumulation of gases leads to a reduction of the culture volume exposed to light and therefore significantly reduces the efficiency of the system. At these gas pockets, it also leads to the drying out of the microalgae and the appearance of a film of dead cells. As a function of the circulation flow rate, which determines the linear velocity of the culture in the tubes, and the type of cultured microorganisms, sedimentation can also occur in the tube. These two effects, adhesion of microalgae and sedimentation, lead to a reduction in the volume of culture exposed to the light; to an evolution of the culture towards a heterogeneous state, because the sedimented cells or the cells fixed to the tube walls escape the continuous culture dilution rate; and to an increase of contamination risks as a result of the development of bacteria and/or protozoa, which develop in the absence of light or at the expense of the dead cells.
It is known in general terms for the permanent cleaning of the interior of pipes of such apparatuses to introduce into the pipes plastic material balls, which are therefore circulated with the culture medium and permanently ensure an agitation of the nutrient medium, together with a cleaning of the tubes as a result of the balls rubbing on their walls.
In order to be effective, this cleaning must be continuous and automatic. Moreover, the use of pumps for circulating the nutrient solution in the tubes and the presence of measuring probes, makes it necessary to limit the cleaning by the balls to the solar receptor only, while excluding the carbonator. Therefore, a cleaning apparatus for such a photobioreactor must contain a system for recycling the balls between the solar receptor inlet and outlet.
The closest prior art in this field is represented by French patent 2,576,034, which describes two constructions for ensuring such a recycling. A description will be given hereinafter with reference to FIGS. 1 and 2, of these two prior art constructions described in the aforementioned patent relating to the circulation of cleaning balls in the solar receptor only. FIGS. 1 and 2 show the photobioreactor with its pipe looped onto itself and incorporating the solar receptor 2 exposed to the radiation 5, as well as the carbonator 6 in series with the aforementioned loop. A pump 10 circulates the liquid nutrient medium throughout the pipe.
In the first construction according to FIG. 1, which is a manual operating mode, use is made of a ball recovery basket 12 having two compartments 14 and 16. The compartment 14 is installed on the outward branch of the photobioreactor circuit and the compartment 16 is installed on the return branch of the circuit. The basket 12 is installed so as to pivot by 180.degree., so that on turning the basket, the compartments 14 and 16 are inverted. During the operation of the installation, the balls are held in the compartment 16 and accumulate there. When the compartment 16 is full, the basket 12 is pivoted by 180.degree. so as to reverse the position of the compartments 14 and 16. The balls in compartment 16 can then be introduced into the circuit by the circulation of the fluid. The reversal of these two baskets 14 and 16 can take place without stopping the circulation of the culture as a result of not shown pipe branches. However, this system involves the opening of the general pipe and consequently the contacting for a certain time of the culture with the exterior. This leads to unacceptable contamination risks of a biological nature such as microorganisms being introduced into the duct.
In the construction according to FIG. 2, there is a specific separation between the solar receptor 2 and the remaining pipe branch in which is located the carbonator 6 by having a branch 7 between the solar receptor inlet and outlet in the following way. A stop grating 18 for the balls is installed on the return branch of the photobioreactor. A special pump 20 for the circulation of the balls is branched onto the outward and return branches of the circuit upstream of the grating 18 with respect to the suspension flow direction. The pump 20 is e.g. a vortex effect pump. In this case, there is no need for the main circuit pump to be a pump which permits the circulation of the balls, so that any random pump type can be used. In view of the fact that the deoxygenating carbonator 6 is not traversed by the balls, because it is not located on the circuit of the pump 20, there is no need for it to allow the circulation of the balls and it is consequently installed directly on the circuit.
In this case, the ball recycling pump 20 operates continuously so that, at the same time as the balls, a significant part of the culture passes directly into the solar receptor without being carbonated. This constitutes a major disadvantage with respect to the efficiency of the photosynthesis reaction and it has also been found that the use of pumps on the solar receptor circuit was very prejudicial to the cells of certain species of microalgae.
For all the above reasons, neither of the prior art arrangements permitting the limitation of the circulation of the balls to the solar receptor functions in a satisfactory manner.